In recent years, the development of number concepts has been conceptualized as a domain specific process that is guided by innate mechanisms and/or principles. This conceptualization implies that number concepts develop differently from other concepts and may be developmentally privileged. However, new research indicates that development of at least one aspect of number- the formation of numerical equivalence classes- is affected by similarity and naming in much the same way as other concepts (Mix, in press; under review). The three projects proposed here seek to investigate further the extent to which domain general processes apply to and explain the development of number concepts. Specifically, this research will (1) use a simple triad matching task to test whether preschoolers' equivalence judgments are affected by variations in numerically irrelevant surface features of the sets; (2) use a longitudinal design to track the development of various numerical skills and concepts in order to establish whether developmental trends suggested in cross-sectional research on number, comparison, and categorization are evident in individual development; and (3) use microgenetic training experiments to test whether experiences found to promote conceptual growth in other domains have similar effects on the formation of number categories. If number concepts develop like other concepts, it would provide important insight into the mechanisms that underlie number concept formation and suggest new avenues for research that have been overlooked in the domain specific framework. It would also increase what is known about the generalizability of the developmental processes that have been posited to explain comparison and categorization in other domains.